The fluorescent dyes Propidium Iodide (PI) and True Blue (TB) are transported along axons in a retrograde direction after microinjection of these substances in brain. Injection of PI or TB in the nucleus tractus solitarius (NTS) followed by fluorescence histochemical processing 3-7 days later revealed that the dyes are transported to the dorsal raphe nucleus. Concurrent immunohistochemical analysis indicated that the dye was present in cells of the dorsal raphe which also contain serotonin. Few other areas in the dorsal mesencephalon were labeled with dye whereas the pons did show some dye. These results suggest that the origin of the serotonergic innervation of the NTS is the dorsal raphe nucleus.